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The Mechanism and Control of Fibrillation in the Mammalian 



Heart. 

 By J. A. MacWilliam, F.K.S. 



(Received June 6, 1918.) 



The results of the present investigation are founded on a very extended 

 study of the subject, carried on from time to time during the past 30 years, 

 in the course of very numerous experiments (hundreds) on the mammalian 

 heart. 



These results establish the conclusion that in fibrillation there is an 

 essential change in the manner of conduction of the excitation process in the 

 cardiac musculature ; the relation of this change to the excitability of the 

 muscle determines the appearance and characters of the different forms of 

 " fibrillar " action that may be observed. The conduction of the excitation is 

 essentially altered, inasmuch as it is propagated along the muscular fibre 

 systems or fasciculi, instead of travelling directly through the muscular 

 substance, without obvious regard to the arrangement of the fibres, as in the 

 normal beat of the heart.* Fascicular dissociation is an essential feature of 

 fibrillation, which, is,, strictly speaking, a condition of " fasciculation " rather 

 than " fibrillation." The essential change in conduction may be induced in 

 very different ways. The state of fibrillation is rendered persistent by a 

 disturbance in the normal relations of conduction time and refractory period 

 in the cardiac musculature, resulting in the establishment of a mechanism of 

 circulating excitations. 



The cat's heart was the one most largely investigated, but those of rabbits, 

 guinea-pigs, rats, etc., were also employed. The heart action was usually 

 examined and recorded with the thorax open, while artificial respiration, by 

 means of a pump or by continuous insufflation of the lungs with oxygen, was 

 maintained. A myocardiograph of the type described by Cushnyf was 

 employed, arterial blood pressure or pulse being often registered at the same 

 time. Intra-cardiac pressure records were often made from the auricles and 

 the ventricles on the principles described by Frank. Anaesthesia was main- 

 tained by chloroform, ether, urethane, morphia, chloretone, paraldehyde, or 

 combinations of these. In a number of experiments the method of decapita- 

 tion was used. The perfused heart was frequently utilised, records being 



* See the electrocardiographic evidence advanced by Lewis and Eothschild, 'Phil. 

 Trans.,' vol. 206, p. 181 (1915). 

 t 'Heart,' vol. 2, p. 1 (1910-11). 



